Extreme erosion and bulking in a giant submarine gravity flow

Sediment gravity flows are ubiquitous agents of transport, erosion, and deposition across Earth’s surface, including terrestrial debris flows, snow avalanches, and submarine turbidity currents. Sediment gravity flows typically erode material along their path (bulking), which can dramatically increase their size, speed, and run-out distance. Hence, flow bulking is a first-order control on flow evolution and underpins predictive modeling approaches and geohazard assessments. Quantifying bulking in submarine systems is problematic because of their large-scale and inaccessible nature, complex stratigraphy, and poorly understood source areas. Here, we map the deposits and erosive destruction of a giant submarine gravity flow from source to sink. The small initial failure (~1.5 cubic kilometers) entrained over 100 times its starting volume, catastrophically evolving into a giant flow with a total volume of ~162 cubic kilometers and a run-out distance of ~2000 kilometers. Entrainment of mud was the critical fuel, which promoted run-away flow growth and extreme levels of erosion.


Figure S1. Bathymetric slope maps of the Agadir Canyon's upper and lower knickpoints. Black is high slope gradient; white is low slope gradient. (A) Slope Gradient map of the upper knickpoint with indicated depth profile (red line) shown in (B). (C) Slope Gradient map of lower Knickpoint with indicated profile (red line) shown in (D). Location of A and C are given in Figure
.

Seismic data
2D high-resolution seismic data were recorded using an 88-channel Geometrics GeoEel streamer with a standard GI-gun (1.7L) as source.Processing was done with Vista Seismic Data Processing Software and included trace binning, filtering, NMO-correction, stacking, and post-stack finite-difference migration.The data has a vertical resolution of ~ 4.5 m (at 80 Hz and 1500 m/s seismic velocity) and a bin size of 2 m (see Figure S2B).The IHS Kingdom software was used to visualize and interpret the seismic data.

Sidescan sonar
Towed Ocean Bottom Instrument (TOBI) is an instrumented vehicle, which is towed close to the bottom of the deep ocean from a ship and uses sound to form detailed images of the sea floor.The TOBI system deploys a deep towed dual sidescan sonar system based around 30 KHz.The range to each side of the nadir (central zone) is 3 km, yielding a total swath width of 6 km.The seabed footprint ranges from about 4 x 7 m close to the vehicle track to 42 x 2 m at far range (see Figure S2C).

Core MSM113-60
Core 60 records 7.8 m of hemipelagic sediment and one gravity flow deposit at 2.6 m depth.The base of Core 60 potentially captures the 72 ka coccolith biozone with appropriate ratios of G.M/E.H species.
However, slightly deeper penetration is needed to confirm the upward trend in species, i.e. to see the transition.Without the longer record, it is difficult to confirm the presence of the 72 ka biozone.As an estimate, we tentatively interpret the coccolith ratios found at the base of Core 60 at 7.88 m hemipelagic depth as the 72 ka biozone.This gives an average sedimentation rate of ~11 cm/kyr, which is similar to nearby Cores 58, 59 and 61.With this age model, the gravity flow deposit at 2.6 m is ~24 ka, which is too young to be Bed 5.

Coccolith assemblages of Bed 5
The coccolith assemblages within Bed 5 are distinct from surrounding event beds (22).They contain both E.Huxlyei (young) and a small proportion of P.Lacunosa (old).This mixture of species indicates

The canyon erosion surface
The widespread erosion surface recorded in all cores in the Agadir Canyon is interpreted to have been generated by the Bed 5 Event.Two lines of evidence support this interpretation.First, Bed 5 is the only large-volume event to record substantial erosion in the Lower Agadir Canyon and Agadir Basin over the past 200 kyr (Fig. 3B; 19,22).Second, the erosion surface throughout the canyon is always draped by Bed 5 deposits (identified by age, composition and coccolith assemblages), even in cores elevated above the canyon floor (Fig. 3B; Core 55).This indicates a close temporal relationship and likely genetic link between the canyon erosion and subsequent emplacement of Bed 5 ~60 ka.
An alternative interpretation is that Bed 5 passively drapes an older erosion surface, hence is not genetically related to the incision.If this were the case, we would expect to record a variety of stratigraphy >60 ka between the erosion surface and Bed 5, particularly in cores on the canyon margins that are more likely to preserve hemipelagic sediments between event beds (e.g.Core 55).However, Bed 5 always immediately overlies the erosion surface in any position within the canyon (see also 19 for canyon margin cores across the lower canyon).Hence, we interpret the erosion surface as being generated by the Bed 5 Event.

Initial failure volume estimates
Whilst the initial failure for Bed 5 is not visible, the field data does rule-out zones of the Southern Tributary catchment as source areas and highlights the Southern Tributary thalweg as a local source of coarse-grained sediment.Figure S5 interprets a shallow-seismic profile across the upper slope, which shows no widespread slope failures.Table S2.Showing a range in the average thickness of thalweg failure in the Southern Tributary, and how this corresponds to variations in the estimated initial failure volume.Note that the range in estimated volumes (0.3-1.5 km 3 ) is relatively minor compared to the total deposit volume down slope of 162 km 3 .

Shear Strength Profiles
. Plot showing the shear strength profiles of five sediment cores in downslope order (see Fig. 1).The cores comprise remobilized sediments with destroyed consolidation profile compared to in-situ seafloor sediments (12).
Figure S3.Age models for cores across the Canyon Head region (Fig. 5), which include GeoB cores situated on the open slope ~10 km away from the tributary thalwegs (after 50).MSM cores dated in this study show similar age trajectories to the GeoB records: MSM32-53 showing a sedimentation rate of ~7 cm/kyr, and MSM113-58, 59 and 61 having slightly higher sedimentation rates of ~10 cm/kyr.Note that Core 59 does not have carbon dates meaning the linear projection likely misses the stepped profile seen in the upper parts of 58 and 61.The error from the E.Huxleyi/G.Mullerae coccolith biozone (72 ka) is ±5000 years.The position of select gravity flow deposits are shown with dashed arrows and an X with corresponding age of emplacement.These deposits are shown as they are potential candidates for Bed 5. Bed 5 is identified in Core 53 as the only gravity flow deposit in the core (dated at ~60 ka).MSM113-61 has two potential gravity flow deposits at 6.2 and 7.3 m depth.The 72 ka coccolith biozone occurs at ~7m, which constrains the ages of these deposits (Bed 5 at 60 ka and C7 at ~75 ka).Note that C7 occurs below the coccolith biozone and must be older than 72 kyr.MSM113-58 only records one gravity flow deposit below the 72 ka coccolith biozone (dated at ~82 ka), which rules it out as beingBed 5.
that the Bed 5 event was sourced from a failure at least 35 m thick (assuming uninterrupted hemipelagic sedimentation between 450 ka and 60 ka) or the parent flow was able to erode down up to 35 m along its pathway.Canyon floor scours up to 100 m deep in the Southern Tributary suggest that flows were powerful enough to erode substantially into the canyon floor.However, it is not possible to determine how much of that scouring was produced by Bed 5 alone.

Figure S5 .
Figure S5.Multibeam bathymetric and backscatter data from the upper Southern Tributary.(A) Swath bathymetry across the upper Southern Tributary thalweg and adjacent catchment.Drainage thalwegs shown as white lines.Core MSM113-61 (Core 61) shown as a red circle.(B) Backscatter image from the swath bathymetry in (A).Note the Southern Tributary canyon floor has a highbackscatter response, indicating a covering of sandy sediments.In contrast, the adjacent continental slopes have a low-backscatter response, which indicates mud-dominated strata (see also Fig. S6 below).As Bed 5 contains a substantial proportion of coarse-grained sand and gravel (e.g.Core 61), it is likely the initial failure included sediments from the canyon floor.

Figure S6 .
Figure S6.Sub-bottom profiles across the southern parts of the Southern Tributary catchment (see Fig. 6A for locations).(A) SBP1 covers the upper slope and is characterized by flat parallel reflectors with deep penetration.These acoustic facies indicate in-situ (undisturbed) hemipelagic muds and ruleout widespread slope failure across this part of the catchment.Note two zones of disturbed sediments: a weakly remobilized unit in the central part of the profile, and a locally thick transparent unit sitting within a minor thalweg on the northern part of the profile.The weakly remobilized unit is subtended by several vertical pipes, each of which are loci for V-shaped deformation (see zoom in).This style of deformation is characteristic of fluidization: injection structures from either water or gas escape (e.g.

Figure S7 .
Figure S7.Plot showing the relationship between thickness of the initial thalweg failure and the resultant volume generated by the failure (Fig. 6D for area).TableS2below provides some scenarios.